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Characterization of Laser Powder Bed Additive Manufacturing and Heat Treatment of Nickel base Superalloy

Additive manufacturing (AM) is advantageous for direct production of complex shaped components based on three-dimensional CAD. As schematically shown in Fig. 1, the laser powder bed AM process consists of the following steps: (1) A thin layer of powder is applied on the substrate. (2) Powder is fused into a cross-section defined by the CAD geometry using raster scanning of a laser beam. (3) The building platform is lowered and the above steps are repeated until the part is completed [Fig.1]. The rapid raster of laser beam subjects the material to repeated cycles of rapid heating, melting, solidification and cooling. As a result, the microstructures produced by laser powder bed AM have unique characteristics when compared to those fabricated by conventional manufacturing processes such as casting or forging. Currently, there is a lack of fundamental understanding on the effect of heat treatment on the unique microstructures produced by AM. Oftentimes, the heat treatment schedules developed for casted or for forged materials are directly applied to the AM materials without considering the response of unique AM microstructure. 

The main objectives of this project are to evaluate (i) the effect of processing parameters, part geometry, and build orientation on the as-build microstructure, and (ii) the effect of post-build heat treatment on microstructure evolution. The full characterization of as-build and heat treated microstructures provides the basis for optimizing heat treatment schedules for mechanical properties.

Nickel base superalloy IN718 parts will be built using an EOSINT M280 - 400 W laser powder bed machine at Edison Welding Institute (EWI). Samples will be built using a wide range of processing parameters, geometry and orientation. The as-build samples will be tehn characterized by Ohio State University using a suite of multi-scale characterization tools. First, a full characterization of spatial variation of microstructure and mechanical property in the sample will be performed using meso-scale tools including optical microscope and micro-hardness mapping. In selected regions of interest, scanning electron microscope (SEM) along with Electron Backscatter Diffraction (EBSD) will indicate the microstructure characteristics including fraction of each microstructure and misorientation between different microstructure grains. At the nano-scale resolution, transmission electron microscope (TEM) will be used to identify the fine gamma prime and double prime (γ’ and γ’’), which are the primary strengthening precipitates for IN718 alloy. The as-build samples will be subject to various heat treatments.  The microstructure development will be characterized with multi-scale characterization methods to understand the effect of heat treatment on the unique microstructures produced by AM.  

This is a new project initiated in Sep. 2013.

Sponsor:  Edison Welding Institute (EWI) – Dr. Shawn Kelly
Graduate Student: Hyeyun Song (PhD)
Collaborator: NASA Marshall Space Flight Center and Applied Optimization Inc.